feat: Add debug logging option for OPA responses

[vdegans]

Description

This PR introduces an optional debug logging configuration for Open Policy Agent (OPA) responses to improve observability during development and troubleshooting.

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Changes

• Added a new configuration field:

  debug = {
      type = "boolean",
      default = false,
      description = "enable debug logging of OPA responses (only for local development)"
  }

• When debug is enabled:

  • Logs the raw OPA response, including HTTP status and body.
  • Logs the evaluated decision result (allow) along with the full decoded response payload.

• Logging is performed using core.log.debug, ensuring no impact on production environments unless explicitly enabled.

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Motivation

OPA policy evaluation can be difficult to debug due to limited visibility into the responses returned by the OPA server. This change provides developers with detailed insights into:

• The raw response returned by OPA
• The parsed decision output used by APISIX

This significantly improves the ability to diagnose misconfigurations, policy errors, or unexpected authorization behavior during local development.

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Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[Baoyuantop]

I don't think this configuration option is necessary. When the log level is set to debug, debug logs will already be printed.